ABSTRACT. An alkaline-based chemical antigen retrieval pretreatment step was used to enhance immunolabeling of disease-associated prion protein (PrP Sc ) in formalin-fixed and paraffin-embedded tissue sections from cattle naturally affected with bovine spongiform encephalopathy (BSE). The modified chemical method used in this study amplified the PrP Sc signal by unmasking PrP Sc compared with the normal cellular prion protein. In addition, this method reduced nonspecific background immunolabeling that resulted from the destruction of the residual normal cellular form of prion protein, and reduced the treatment time compared with the usual autoclave pretreatment step. Immunolabeled PrP Sc was thereby clearly detected in the myenteric plexus of the ileum in naturally occurring BSE cattle.KEY WORDS: antigen retrieval, BSE, cattle, immunohistochemistry, prion.

ABSTRACT. Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative disorder of cattle characterized by accumulation of the disease-associated prion protein (PrP Sc ) in the central nervous system (CNS). The immunohistochemical patterns and distribution of PrP Sc were investigated in the CNS, brains, and spinal cords of 7 naturally occurring BSE cases confirmed by the fallen stock surveillance program in Japan. No animals showed characteristic clinical signs of the disease. Coronal slices of 14 different brain areas in each case were immunohistochemically analyzed using an anti-prion protein antibody. Immunolabeled PrP Sc deposition was widely observed throughout each brain and spinal cord. Intense PrP Sc deposition was greater in the thalamus, brainstem, and spinal cord of the gray matter than in the neocortices. The topographical distribution pattern and severity of PrP Sc accumulation were mapped and plotted as immunohistochemical profiles of the different brain areas along the caudal-rostral axis of the brain. The distribution pattern and severity of the immunolabeled PrP Sc in the CNS were almost the same among the 7 cases analyzed, suggesting that the naturally occurring cases in this study were at the preclinical stage of the disease. Immunohistochemical mapping of the PrP Sc deposits will be used to clarify the different stages of BSE in cattle.